1. Field of the Invention
The invention relates to the field of the cleansing or purification and deodorizing of gas effluents.
More specifically, the invention relates to a method that enables a significant abatement of the level of these effluents contained in the pollutants that cause olfactory nuisance, enabling notably an abatement of the concentration in sulfurous compounds and nitrogenous compounds.
Purification stations are the source of many types of olfactory nuisance, chiefly ammonia, organic nitrogenous compounds (such as methylamine, indole and scatole, etc.), sulfurous compounds (hydrogen sulfide, thiols, disulfides) as well as various aldehydes and ketones.
Furthermore, human beings have relatively low and even very low perception thresholds with respect to the volatile sulfurous and nitrogenous species responsible for odors. This results in considerable discomfort to people living in the neighborhood as well as to passers-by.
2. Description of the Prior Art
There are various techniques known in the prior art for the purification and deodorizing of gas effluents.
In one of these techniques, the gas effluents are put through a succession of washing columns or towers, each containing a liquid that has the task of trapping one type of organic pollution. The installation that implements such a technique has at least three towers: one with the task of trapping acidic pH soluble nitrogenous pollutants, another one with the task of trapping low basic pH soluble sulfurous pollutants and yet another one with the task of trapping highly basic pH soluble sulfurous pollutants.
While such installations enable high speeds of passage of gas effluents into the towers, they have the major drawback of taking up a great deal of space. It is sometimes impossible to make use of such installations.
Other techniques consist in trapping the odoriferous molecules in a liquid or within a solid material and in permitting the degradation of these modules by a biomass. Thus bioscrubbers of the type described in the document EP-A-0 218 958 are known in the prior art. In such installations, the treated gas is first of all placed in contact with a biomass in an aqueous phase so as to transfer a part of the pollutants in the gas phase to the biomass. This biomass is then conveyed to a reactor in which the pollutants are degraded.
However, such installations enable the elimination only of constituents that are soluble at the pH of the aqueous phase used. They do not enable the elimination of pollutants that are not soluble in water at the pH considered.
There also exists known biowashers wherein the pollution is trapped in a washing water and then degraded in a second reactor by an appropriate biomass, and biofilters in which the pollution is trapped by a material on which it is adsorbed and/or absorbed so as to enable its subsequent degradation by bacteria.
It is more specifically to the technique of biodeodorization on biofilters that the invention pertains.
Thus, in the prior art, there is a known method for the deodorizing and purification of gas containing biodegradable pollutants consisting in making the gas pass through a support formed by layers of peat in the presence of micro-organisms, and in wetting said layers with aqueous solutions that bring complementary additional nutrients. This technique is described in the document FR-A-2 519 122. The biomass used in such a technique consists essentially of heterotrophic bacteria using, as a carbon source and nitrogen source, the organic material present in peat and in a nutrient sprinkling solution that contains organic carbon.
Other methods using adsorption on materials such as active carbon are known. However, to implement such methods at high delivery rates, it is necessary to build costly installations.
Although such techniques make it possible to obtain efficient deodorizing results, they have a certain number of drawbacks.
First of all, they do not permit treatment ensuring that nuisance thresholds will be obtained when the effluents are highly charged with pollutants. Thus, the performance characteristics of such biofilters drop when the hydrogen sulfide content exceeds 15 to 20 milligrams per liter.
Secondly, such techniques have the drawback of not enabling very high speeds of passage. This makes it necessary to configure the filters so that they have a large filtration surface area. In practice, the speeds of treatment that can be achieved with such installations do not exceed 100 meters per hour.
Finally, biofilters using layers of peat also have the drawback of reacting slowly to sudden changes in load, and this limits their range of use.
There are also known biofilters implementing a heterotrophic biomass on an inert support such as those described in the documents EP-A-94 573 or EP-A-224 889.
These biofilters have the drawback wherein they can eliminate only one type of pollution at a time (for example H.sub.2 S) and are capable of working only with low speeds of treatment (lower than 100 meters an hour).
The aim of the present invention is to provide a method of bio-deodorization that does not have the drawbacks of prior art techniques.
In particular, one of the aims of the present invention is to describe a method enabling the treatment of gas effluents that could be highly charged with pollutants.
Another aim of the present invention is to provide a method that can be used to cope efficiently with sudden and frequent variations in the pollution load contained in the effluents without prompting any major reduction in the rates of abatement of this pollution.
Another aim of the invention is to provide an installation for the implementation of such a method having sizes that are far smaller than those of the prior art installations, including biofilters using layers of peat.